1. Field of the Invention
This invention relates to a conversion sleeve and an optical adapter to be used for interconnecting optical connectors of the optical fiber cable ends which are used in optical communications, particularly the conversion sleeve and the conversion adapter to be used for interconnecting optical connectors containing ferrules having different diameters.
2. Description of the Prior Art
In an optical connector which is used to facilitate the connection and the disconnection of two optical fibers or of an optical fiber and other optical element, like the SC type optical connector, for example, the predominantly used system comprises fitting one end of an optical fiber into an axial through-hole of a ferrule and inserting two ferrules thus prepared into an alignment sleeve of an optical adapter through the opposite ends thereof to abut end faces of the ferrules against each other.
The use of the sleeve for aligning the ferrules each having an optical fiber inserted and fixed therein by abutting end faces thereof against each other includes the case where the optical connectors having the same ferrule diameter are mutually connected and the case where the optical connectors having different ferrule diameters are mutually connected. In the latter case, a conversion sleeve is used. The modes of conversion sleeves are divided into two main classes; a precision sleeve having no slit and an elastic sleeve having a slit (split sleeve).
AS an elastic sleeve, the structures as shown in FIG. 1 is known in the art, for example, as disclosed in published Japanese Patent Application, JP 9-90169 A.
The conversion sleeve 1 shown in FIG. 1 is a split sleeve comprising a large diameter part 2 having a large diameter through-hole “a”, into which a large diameter ferrule A1 having an optical fiber B inserted and fixed therein is fitted, and a small diameter part 3 formed by reducing a diameter in one end portion of the large diameter part so as to form a tapered diameter part 4 and having a small diameter through-hole “b” into which a small diameter ferrule A2 having an optical fiber B inserted and fixed therein is fitted, the sleeve having a slit 5 formed in the longitudinal direction thereof so as to elastically hold the ferrules A1 and A2 having different diameters. The large diameter part and the small diameter part are integrally formed of metal or plastic.
A conversion sleeve is required to have lower loss of light, when ferrules having the different diameters, for example 2.50 mm and 1.25 mm, are connected. Accordingly, it is required that the deviation of one center from the other (offset) of ferrules should be 1 micron or less.
In the case of the precision sleeve, the axial deviation and the accuracy of the inner diameter is significant. Since the highly accurate processing is difficult, it poses the problem that the processing cost becomes high.
On the other hand, in the case of the elastic sleeve, although the accuracy of the inner diameter is not so significant, crookedness (the state where the SC ferrule and the MU ferrule are not aligned in a completely straight line) will arise by elastic deformation of the sleeve. In this case, the problem of deviation in the contact position will arise. Furthermore, if the length of the leading part of the ferrule changes due to re-polishing or other causes, it will pose the problems that the deviation in the contact position will occur similarly, the offset becomes large, and thus the connection loss becomes large. That is, its weak point is that the contact position is uncertain.
Furthermore, in the case of an elastic conversion sleeve, since the difference in the inner diameter between the opposite sides results in three-dimensional profile with a step, this step portion exerts great influence on the difference in the degree of deformation between of the large diameter part and the small diameter part of the sleeve. Therefore, in order to decrease this difference as small as possible, as an angle of the step portion 45° is adopted instead of 90° in many cases. The sleeve made of such a material as phosphor bronze and a synthetic resin has this profile. When the angle is 45°, however, since the taper at the leading end of the SC ferrule has the angle of 60°, the tip of the ferrule 10 will abut against the tapered part 4 of the conversion sleeve 1 when a connector is inserted (see FIG. 1). That is, the portion at which the tapered diameter part 4 of the conversion sleeve 1 having the angle of 90° comes into contact with the tapered part of the tip of the ferrule having the angle of 60° is the PC contact position of the ferrule. In this case, if the tapered part of the tip of the ferrule having the angle of 60° is always the same length, the PC contact position will be fixed. However, the length of the tapered part having the angle of 60° varies with the PC polishing procedure of the ferrule and the number of repetition of polishing. Accordingly, the PC contact position varies with the connector to be used. As described above, in the elastic conversion sleeve which is provided with a slit to acquire suitable retaining force, there is a problem that the PC contact position will vary with the connectors to be used.
Although the optical fiber which is retained and fixed in the ferrule is positioned along the axis of the ferrule with high accuracy, usually it has practically slight eccentricity to the outer diameter of the ferrule. Therefore, there is a problem that, when the alignment sleeve rotates, it causes a deviation between the axes of the ferrules (the cores of the optical fibers) and the change in the optical characteristics.
Particularly in the optical adapter (conversion adapter) using the conversion sleeve, since the inside of the conversion sleeve has the profile of a hole with a step as mentioned above, it was very difficult to bore two holes from opposite sides so as to coincide the axes thereof with each other with high accuracy in the order of 1 micron or less.
Moreover, even when the optical connectors each having the optical fiber retained and fixed in the center of the ferrule thereof are connected by using the conversion adapter mentioned above, if the conversion sleeve rotates within the adapter each time the optical connectors are connected, the so-called axial deviation in the connecting point becomes larger. This poses the problem that the connection loss becomes larger.